Integrity check for fire alarm circuits



Nov. 10, 1970 J. T. MORROW ETAL INTEGRITY CHECK FOR FIRE ALARM CIRCUITSFiled June 26, 1967 #MBX) SOURCE I E ,ll

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DETECTORS SUPERVISORY INDICATOR LAMP IN VENTORS BY W.

J. T. MORROW AND E. ROWLEY, JR.

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THEIR ATTORNEY icut Filed June 26, 1967, Ser. No. 648,684. Int. Cl.G081) 29/00 US. Cl. 340-409 13 Claims ABSTRACT OF THE DISCLOSURE Anormally inactive alarm circuit is energized by an alternating currentand comprises two resistors and an alarm device in series. The alarm canbe sounded by the operation of an alarm detector which shunts one of theresistors, and thereby increases current flow through the alarm device.A capacitor in a separate supervisory circuit is intermittently chargedthrough a diode from a source of alternating current, and is subject todischarge each time a supervisory signal is generated across a diodewhich is connected in multiple With the other resistor in the alarmcircuit. If a charge is permitted to build up on the capacitor, asupervisory alarm is activated.

BACKGROUND OF THE INVENTION This invention relates to integrity checkingsystems for alarm circuits, and it more particularly relates to anintegrity checking system having solid state supervisory controlapparatus.

An alarm circuit to be supervised is generally a closed circuitenergized by a relatively low current for checking integrity of thecircuit. The detection of an alarm condition increases the current inthe alarm circuit materially by shunting a series resistor, and suchincrease either operates an audible alarm directly, or actuates aswitching device such as a relay, which in turn operates the alarm. Itis desirable to continuously check the integrity of the alarm circuit,including all of the elements required to be actuated in response to thedetection of a condition for which an alarm should be given.

SUMMARY OF INVENTION The system according to the present invention has asupervisory system for continuously checking the integrity of an alarmcircuit. Such alarm circuit is normally energized from an alternatingcurrent source and includes in series two resistors and a normallyinactive audible alarm device, such as a bell. The detection of an alarmcondition, such as the actuation of a fire detector, for example, shuntsone of the resistors, which materially increases current in the alarmcircuit and thereby actuates the bell.

The supervisory system senses current in the alarm circuit, and if thecurrent in the alarm circuit is above a predetermined value, nosupervisory alarm is given, but if current in the alarm circuit dropsbelow this value, due to an open or ground fault in the wiring or in analarm detector or alarm signal, a supervisory alarm is rendered active.This mode of operation is accomplished by super visory controls governedby sensing the potential across an inverse parallel arrangement ofdiodes connected in multiple with the other of the resistors in thealarm circuit. When current in the alarm is normal, there is sufiicientvoltage drop across said another resistor to forward bias the diode tothe break point for a portion of each alternate half cycle of thesupervisory current, thus clamping the voltage across said anotherresistor to the forward 3,54%,42 Patented Nov. 10, 1970 ice voltage dropof the diode. When the alarm is active, the relatively heavy current inthe alarm circuit is passed by the inverse parallel arrangement ofdiodes and the voltage across said another resistor is again clamped tothe forward voltage drop of the diodes, thus protecting the supervisorycontrol apparatus against damage and against improper operation inresponse to the high value of current in the active alarm circuit.

The supervisory signal comprises half wave pulses which operate a switchto intermittently discharge at capacitor, the capacitor having aseparate circuit by which it is intermittently charged. Should thisswitch cease to operate, as by failure to generate a supervisory signalof sufiicient amplitude, the capacitor does not become discharged and asupervisory alarm is given. A timing circuit involving another capacitoris employed to delay actuation of the supervisory alarm so that thesupervisory alarm is not given for a momentary interruption of thesupervisory signal.

An object of the present invention is to provide a substantiallyfail-safe and economical supervisory alarm system for checking integrityof an alarm circuit using solid state switching means for control of asupervisory alarm device.

Another object of the present invention is to protect the solid stateswitching means when the alarm circuit is rendered active by use of atleast one diode in a circuit shunting input to the solid state switchingmeans.

Other objects, purposes and characteristic features of the presentinvention will be in part obvious from the accompanying drawings and inpart pointed out as the description of the invention progresses.

In describing the invention in retail reference is made to theaccompanying drawing which illustrates diagrammatically one embodimentof the present invention as applied to supervisory checking of theintegrity of a fire alarm system.

With reference to the drawing, an alarm bell 10 is connected in an alarmcircuit for manifesting .the detection of an alarm condition, such, forexample, as the detec tion of a fire by closure of a contact of anormally open fire detector 11. A supervisory alarm bell 12 is connectedin a supervisory alarm circuit for manifesting failure to check theintegrity of the alarm circuit. A supervisory indicator lamp 13 isprovided for also indicating failure to check integrity of the alarmcircuit, and a manually operable switch 14 is provided for silencing thesupervisory alarm bell.

The alarm circuit is energized from a suitable source of alternatingcurrent having its supply terminals designated as (BX) and (CX)respectively. This circuit comprises in series resistor 15, bell 10,contact 16 of switch 14 in its right hand position, and resistor 17.Thiscircuit normally has a relatively low current flow, particularlybecause of the relatively high resistance of resistor 15. The low levelcurrent in the alarm circuit flows through the alarm bell 10, but thisbell is not actuated because of the relatively low current level. When adetector 11 is actuated to close its contacts, the resistor 15 isshunted, and the bell 10 is rendered active by the flow of relativelyheavy current in the alarm circuit.

The circuit for the supervisory bell 12 is normally open because itincludes in series a SCR switch 18, which is normally turned off. Thecircuit for the operation of the supervisory bell 12 is energized from asource of the nating current also having its supply terminals designatedas (BX) and (CX). This circuit includes a half wave rectifier 19,winding of bell 12 and SCR switch 18.

The turning on of the SCR switch 18 in accordance with the applicationof a potential to its gate is in response to potential across acapacitor which in turn has its charge determined by a capacitor 21. Thecapacitor 21 has a charging circuit including diode 19 and resistor 22.Thus the capacitor 21 becomes charged by half wave pulses from thealternating current source for the supervisory circuit. This source mayor may not be the same as the source for energization of the alarmcircuit, and thus it is not necessary that the power supply used for thesupervisory circuit organization be in phase with the power supply usedfor the energization of the alarm circuit.

A transistor Q1 is provided for intermittently discharging the capacitor21 and thus normally prevents the capacitor 21 from charging to apotential sufficient to turn the SCR switch 18 on. The transistor Q1 hasits collectoremitter circuit connected across the capacitor 21 and itsbase-emitter circuit connected through a resistor 23 across resistor 17in the alarm circuit and across an inverse parallel arrangementincluding diodes 24 and 28. If the transistor Q1 and the diode 24 are ofthe same material, it may be desired to use two diodes in series toreplace diode 24 in order to obtain the best operating characteristicsof the system.

The diode 24 is forward biased by the voltage drop across the resistor17, and when the alarm circuit is in its normal condition, with arelatively low current flowing, this voltage drop is sufficient torender the diode 24 conductive above its break point. Thus a sufiicientvoltage drop appears across the base-emitter circuit of transistor Q1during each alternate half cycle of the energization of the alarmcircuit of a particular polarity to turn the tran sistor Q1 on, and thusto discharge the capacitor 21. If the capacitor 21 is dischargedintermittently by this discharge circuit organization, it cannot applyany substantial charge to the capacitor 20 through resistor 25. Thus theswitch 18 is normally maintained in its turned off condition. Any chargewhich is applied to capacitor 20, is dissipated through resistor 26.

If the current falls below a predetermined value, the voltage developedacross resistor 17 will be of too low a value to turn on transistor Q1sufliciently to cause complete discharge of capacitor 21 on eachalternate halfcycle. Capacitor 21 thus becomes fully charged through thediode 19, and it in turn charges capacitor 20 through resistor 25. Aftercapacitor 20 has had time to become charged, positive potential on thegate of switch 18 turns the switch 18 on and the supervisory bell 12 isrendered active. Also the indicator lamp 13 is illuminated because it isconnected in multiple with the bell 12 through contact 27 of switch 14.The sounding of the supervisory bell 12 attracts the attention of anoperator, and the operator can silence the bell but still maintain theindicator lamp 13 illuminated by operation of the switch 14 to its lefthand position.

With the switch 14 in its left hand position a connection is made in thealarm circuit through contact 16 to bypass the resistor 17. Currentflowing through the lamp 13, contact 27 of switch 14 in its left handposition and resistor 1'7 in the alarm circuit, provides suflicient biasfor the transistor Q1 to cause transistor Q1 to be active and thus toshunt the capacitor 21 each time it is energized through the diode 19.Thus the capacitor 20 becomes discharged, and the switch 18 is turnedoil to silence the supervisory bell 12. The indicator lamp 13 remainson, however, to indicate to an operator that the alarm has been soundedand that the system has not yet been restored to its normal condition byoperating the switch to its right hand position.

It is thus provided that with the manual switch 14 in its normalposition, current in the alarm circuit passes through resistor 17, andwith the manual switch 14 in its left hand position, the current forlamp 13 passes through the resistor 17. Either current renders thetransistor Q1 active to intermittently discharge the capacitor 21, andthis holds gate voltage below the turn on point for the switch 18.

Should the lamp 13 become burned out, or removed, the supervisory bell12 cannot be silenced by operating the manual switch 14 to its left handposition, thus calling attention to this condition.

Resistor 17 in the alarm circuit is shown as being variable so that thesensitivity of the supervisory circuit can be adjusted to determine theminimum current required in the alarm circuit to prevent activation ofthe supervisory bell 12. Diode 28 is preferably used in multiple withresistor 17 to provide a low resistance path for the other half cycleduring alarm conditions as compared to the half cycle current throughdiode 24.

Having thus described one embodiment of an integrity checking system foralarm circuits, it is desired to be understood that this form isselected to facilitate the disclosure of the invention, rather than tolimit the number of forms the invention can assume. While the inventionhas been described in its preferred embodiment, it is to be understoodthat the words which have been used are words of description rather thanof limitation, and that changes within the purview of the appendingclaims can be made without departing from the true spirit and scope ofthe invention in its proper aspects.

What is claimed is:

1. A supervisory indication system for checking the integrity of analarm circuit which is normally energized by alternating current and hasa series an audible alarm device and a plurality of resistors wherein alow level alternating current is normally present and is shifted to ahigher level current by shunting one of the resistors when an alarm isto be sounded, and wherein the improvement comprises,

(a) at least one diode connected in parallel with the other resistor forgenerating a pulsed direct current supervisory signal across the otherresistor and diode in parallel characteristics of the presence ofcurrent in the alarm circuit when the voltage drop across said otherresistor is sufficient to forward bias the diode, thus clamping thesupervisor signal to the forward voltage drop of the diode,

(b) a first capacitor for storage of a control signal,

(c) means independent of the supervisory signal for intermittentlycharging the capacitor,

(d) means responsive to the pulsed direct current supervisory signal forintermittently discharging the capacitor, and

(e) supervisory alarm means governed by the control signal formanifesting when current in the alarm circuit is below a predeterminedvalue.

2. The invention according to claim 1 wherein the supervisory alarmmeans comprises a normally inactive supervisory alarm device controlledby a solid state switching means, and means is provided for turning theswitching means on to activate the supervisory alarm device in responseto an abnormally high control signal stored by the first capacitor.

3. The invention according to claim 2 wherein the supervisory alarmmeans includes an indicator lamp that is energized in response to theturning on of the solid state switching means.

4. The invention according to claim 3 wherein control means including amanual control switch is provided for permitting operation of thesupervisory alarm device and the energization of the indicator lamp whenthe switching means is in one position, and for energizing the indicatorlamp and preventing operation of the device when the switching means isin another position.

5. The invention according to claim 4 wherein the control means permitsthe supervisory alarm device to be silenced by actuation of theswitching means to said another position only provided there is a closedcircuit through the indicator lamp.

6. The invention according to claim 5 wherein the con- F trol means iseffective to activate the supervisory alarm device upon the indicatorlamp becoming open circuited when the switching means is in said anotherposition.

7. The invention according to claim 1 wherein means for discharging thefirst capacitor comprises a circuit through a solid state switchingmeans, and control means is provided for turning the solid stateswitching means on intermittently in response to the pulsed directcurrent supervisory signal.

8. The invention according to claim 7 wherein the solid state switchingmeans is a transistor having its baseemitter circuit connected inmultiple with said another resistor.

9. The invention according to claim 1 wherein timing means including asecond capacitor is provided for delaying the response of thesupervisory alarm means to a control signal stored by the firstcapacitor.

10. A supervisory indication system for checking the integrity of analarm circuit which is normally energized by alternating current and hasin series an audible alarm device and a plurality of resistors wherein alow level current is normally present and is shifted to a higher levelcurrent by shunting one of the resistors when an alarm is to be sounded,and wherein the improvement comprises,

(a) a diode connected in parallel with the other resistor for generatinga pulsed direct current supervisory signal across the diodecharacteristic of the presence of current in the alarm circuit, wherebythe amplitude of the signal is limited when the diode is forward biasedbeyond its break point, and (b) supervisory alarm means including anormally off solid state switching means and governed by the supervisorysignal for manifesting when current in the alarm circuit is below apredetermined level. 11. The invention according to claim 10 whereingating circuit means is provided for said solid state switching means toturn the solid state switching means on when the supervisory signal isbelow a predetermined value.

12. The invention according to claim 11 wherein manually operableswitching means is provided for normally connecting said anotherresistor in the alarm circuit and at other times connecting said anotherresistor in another circuit for lowering the gate potential on the solidstate switching means to a point where the solid state switching meansis maintained turned oil.

13. The invention according to claim 12 wherein said another circuitincludes an indicator lamp.

References Cited UNITED STATES PATENTS 3,010,100 11/1961 Muehter 340409THOMAS B. HABECKER, Primary Examiner U.S. Cl. X.R. 340-1 76

